Traditional wheelchairs allow and, indeed, encourage users to passively sit and avoid using their muscles.
Traditional walkers assist users who have difficulty in walking without a cane, or other device, or unassisted by others. Yet, such traditional walkers have no seats associated therewith whereby a user can sit at will. Thus, the user of a traditional walker is limited to walking and exercising in areas dependent on the seating available should the user desire or be required to sit while walking around.
Thus, both traditional wheelchairs and conventional walkers inherently discourage their respective users from exercising.